HCV is a single-stranded RNA virus that is a member of the Flaviviridae family. The virus shows extensive genetic heterogeneity as there are currently seven identified genotypes and more than 50 identified subtypes. In HCV infected cells, viral RNA is translated into a polyprotein that is cleaved into ten individual proteins. At the amino terminus are structural proteins: the core (C) protein and the envelope glycoproteins, E1 and E2. p7, an integral membrane protein, follows E1 and E2. Additionally, there are six non-structural proteins, NS2, NS3, NS4A, NS4B, NS5A and NS5B, which play a functional role in the HCV life cycle. (see, for example, B. D. Lindenbach and C. M. Rice, Nature. 436:933-938, 2005). NS5B is the RNA polymerase or replicase of the virus and is responsible for replication of both positive and negative-strand genomic RNA during the viral replicative cycle. NS5B plays an essential and critical role in viral replication, and a functional NS5B replicase is required for HCV replication and infection. Thus, inhibition of NS5B RNA-dependent polymerase activity is believed to be an effective way of treating HCV infection.
Infection by HCV is a serious health issue. It is estimated that 170 million people worldwide are chronically infected with HCV. HCV infection can lead to chronic hepatitis, cirrhosis, liver failure and hepatocellular carcinoma. Chronic HCV infection is thus a major worldwide cause of liver-related premature mortality.
The current standard of care treatment regimen for HCV infection involves interferon-alpha, alone, or in combination with ribavirin and a protease inhibtor. The treatment is cumbersome and sometimes has debilitating and severe side effects and many patients do not durably respond to treatment. New and effective methods of treating HCV infection are urgently needed.